A largely accidental battle, pitting Robert E. Lee against George B. McClellan, became the single deadliest day in America's history and changed the course of the Civil War.

The day of Antietam—September 17, 1862 — was like no other day of the Civil War. “The roar of the infantry was beyond anything conceivable to the uninitiated,” wrote a Union officer who fought there. “If all the stone and brick houses of Broadway should tumble at once the roar and rattle could hardly be greater … and amidst this, hundreds of pieces of artillery, right and left, were thundering as a sort of bass to the infernal music.” Over the course of 14 hours of this unceasing roar and rattle, 22,700 Northerners and Southerners were killed, wounded, or listed as missing—the worst one-day toll of the war, indeed the worst loss of life in a single day in America’s history.

Yet at the end of that indescribably bloody September 17, neither side had won and neither had lost. A Northern war correspondent called it, optimistically, “partly a success; not a victory, but an advantage had been gained.” A Southern correspondent said the Confederates grudgingly accepted it as a drawn battle only “because they had not in their usual style got the enemy to running.” Militarily it was a day of wrenching missed opportunities. But in fact no battle of the Civil War—not Gettysburg, not Vicksburg, not Missionary Ridge—was in the end more meaningful than Antietam. Neither of the two army commanders—Robert E. Lee and George B. McClellan—had planned to fight this battle where and when they did. Two weeks earlier, Lee had led his Army of Northern Virginia across the Potomac into western Maryland to fight a showdown battle on ground of his choosing in Maryland or Pennsylvania. General Lee, according to the notes of a postwar interviewer, said he intended to “have had all my troops reconcentrated on the Md. side, stragglers up, men rested & I intended then to attack McClellan, hoping the best results from state of my troops & those of enemy.” General McClellan was more inclusive and less definite: “Again I have been called upon to save the country,” he wrote his wife on September 5; “the case is desperate, but with God’s help I will try unselfishly to do my best & if he wills it accomplish the salvation of the nation.”

Lee was on a winning streak. In the late spring, in the titanic Seven Days Battles on the Virginia Peninsula, he drove McClellan and his Army of the Potomac away from the gates of Richmond. By September Lee was at the gates of Washington, having roundly defeated a second Union army under John Pope in the Second Battle of Bull Run. Lee cast his eyes north. He regarded a short war as the Confederacy’s best route to victory, and he wrote Jefferson Davis that now was the time to propose to the United States “the recognition of our independence … when it is in our power to inflict injury upon our adversary.” He intended to deliver that injury on Northern soil against the North’s principal army. He hoped that his army, simply by the act of invading Northern soil, would influence the approaching midterm elections and increase support for a Northern peace party.

That first week of September found the Lincoln administration caught up in a command crisis. In August McClellan had been ordered up from the peninsula to combine his army with Pope’s to counter Lee’s advance. But the Young Napoleon (as the press dubbed McClellan) clung to his Peninsula Campaign. He despised John Pope and willfully dragged his feet, holding back two army corps that could have made the difference in Pope’s fight at Bull Run. This conduct infuriated the president. “Unquestionably he has acted badly toward Pope!” he told his secretary. “He wanted him to fail. That is unpardonable.” But he added, “He is too useful now to sacrifice.” Against his cabinet, which petitioned him to sack McClellan, against even his own best instincts, Lincoln replaced Pope with McClellan and gave him command of the combined armies. The deciding factor was the demoralized state of the troops. It was feared that just now the army would not fight for anyone but the Young Napoleon.

The modern term “charismatic” nicely describes George McClellan. His men admired him, and he cultivated their admiration with grand reviews and ringing proclamations. He was a scholar of the Napoleonic Wars (studied in the original French) and struck Napoleonic poses. Yet on the peninsula he was not to be seen on the battlefields. “Curiously enough,” wrote soldier-historian Francis W. Palfrey, “there was almost always something for McClellan to do more important than to fight his own battles.” The pending battle in Maryland would be a first for the Young Napoleon.

McClellan’s particular skill was military organization, now apparent in his gathering up of the disparate commands from the disastrous Second Bull Run campaign, reorganizing and refitting them, and restoring morale. In a remarkably short time the reconstituted Army of the Potomac was ready to fight again. Reports placed the Confederate army at Frederick in Maryland, and McClellan set his own army in motion to follow. He moved with great circumspection. By his accounting he was (once again) heavily outnumbered—the Confederates, he claimed, were 120,000 strong, surpassing his army by at least 30,000 men. McClellan had assumed this underdog role from the moment he took command of the Army of the Potomac in August 1861, and that stance had shaped his every strategic and tactical decision since. But it was and had always been a grand delusion. It was his army that was invariably the stronger, by a factor of two or even three. Such was the case in Maryland. At Antietam McClellan had roughly two and a half times Lee’s manpower. He would wage war against a phantom Confederate army.